Letter size is the standard size for letters (thus, the name), newsletters and magazines (finished/folded size), copy machine paper, and desktop printing papers. The actual dimensions depend on where you live.
Newsletters are typically printed on tabloid size sheets of paper and then folded in half to create a letter size publication. Brochures, especially those printed from a desktop printer, often start out as letter size and are then folded to create a tri-fold brochure. After trimming the actual dimensions of a letter size publication may be slightly smaller.